First things first - I have sourced a CRT! It was delivered for free and works great. Since the PS/2 System 50 has a weird VGA port, which was common in this time period, I had to break a pin off the monitor cable.
Even though the plastic colours do not match, I prefer it greatly over the LCD I used before.
I also received a USB floppy drive in the mail. I tried both MS-DOS and an IBM initialization disk image I found, but it kept dropping me into BASIC. After more troubleshooting, it turns out the floppy drive in the unit is broken. It's a common problem, and should be fixable by replacing its capacitors. When trying to get the drive apart, however, I found out that a few of the screws were completely stuck, and made out of a very soft metal which couldn't handle my screwdriver. I will probably have to drill them out and find replacements.
When troubleshooting, I took the HDD out of the computer, and it no longer booted into BASIC. So, it seems that the BASIC interpreter is stored on the disk, not on the ROM like I previously thought. It turns out it's only 20MB, not 40.
I looked into the development of an MCA card. As far as I can tell, it's not been done before by a hobbyist. I was able to find 2 PDFs on the physical specifications of MCA cards, as well as how the protocol operates. It seems like it would be quite easy to design a card. For me, the hard part is designing the edge connector - the physical specifications are hard to read and contain ambiguity. There's already an MCA card in there, which I may have to measure. I'd like to make an eagle library with the connector, and then I can develop as many boards as I want.